A slit die is also called a spinneret, die, slot-die or dies. A slit die is used to discharge a coating liquid from a slit-like discharge opening formed toward outside in a lip gap formed between a pair of lips facing each other, to form a coating film on a surface of a substrate facing the discharge opening with a clearance formed between the discharge opening and the substrate. Such slit dies are widely used. When a coating film is formed on a substrate by a slit die, the slit die and the substrate are moved relatively to each other.
As an example, a color filter having a fine lattice pattern of three primary colors formed on a glass substrate is described below. A color filter is produced by coating a glass substrate with coating liquids of black, red, blue and green one after another. The color filter production process may include steps of forming a coating film of photoresist, then patterning by photolithography, and forming poles for forming the space of the liquid crystal to be injected between a color filter and an array substrate, and may also include a step of forming an overcoating film for reducing the ruggedness of the surface.
For this kind of film forming process, spinners have been popularly used for such reasons that the viscosity of the coating liquids used is less than tens of mPa·s and that uniform films can be easily formed. However, recently it is desired to reduce consumption of expensive coating liquids, and it is difficult to enlarge the equipment in response to the increasing use of larger substrates to be coated. So, die coaters using slit dies are being used to substitute the spinners.
One of the important functions required for the slit die is to form a uniformly thick coating film. Especially the slit dies used for producing members for displays such as color filters for color liquid crystal displays and back plates and the like for plasma displays are required to use longer components in response to the yearly expanding screens of displays, and the requirement for the uniformity in the thickness of a coating film over a wide coating area becomes severe. Recently, it is required to achieve a coating thickness accuracy as very severe as 3% or less as the maximum deviation from the mean thickness of the coating film.
To meet this requirement, it is necessary that when a die is assembled, the lip gap usually set at 0.05 mm to 0.7 mm is uniformly formed with a deviation in the order of sub-microns. However, publicly known conventional slit dies cannot achieve a lip gap accuracy in the order of sub-microns owing to their structures, and the above-mentioned coating thickness accuracy could not have been achieved.
The problems of the conventional slit dies are concretely explained below. FIGS. 11, 12 and 13 show the transverse sectional views of respectively different publicly known slit dies 201, 301 and 401.
In FIG. 11, the slit die 201 comprises a die hopper 205, a right lip 202 and a left lip 203. The right lip 202 and the left lip 203 are positioned to face each other with a lip gap 212 formed between them. The top face of the right lip 202 and the top face of the left lip 203 respectively contact the bottom face of the die hopper 205, and are respectively attached to the die hopper 205 for integration by means of bolts 206 and 207.
The lip gap 212 has a lip gap width L. This slit die 201 is disclosed in JP 10-264229 A. The slit die 201 with this constitution needs such complicated assembling work in which both the lips 202 and 203 must be positioned against the die hopper 205 while the lip gap width L is measured in the longitudinal direction of the lip gap 212 (in the direction perpendicular to the paper surface). This assembling work does not practically allow a lip gap accuracy in the order of sub-microns to be achieved.
In FIG. 12, the slit die 301 comprises a right lip 302, a left lip 303 and a shim 304. Both the lips 302 and 303 are combined for integration by a bolt 305 with the shim 304 kept between them. A lip gap 312 is formed by the thickness St of the shim 304.
The lip gap 312 has a lip gap width L. This slit die 301 is disclosed in JP 2001-46949 A or JP 2001-191004 A. In the slit die 301 with this constitution, the lip gap width L of the lip gap 312 is equal to the thickness St of the shim 304, irrespectively of the assembling method. Therefore, to achieve a lip gap accuracy in the order of sub-microns, the thin shim 304 having a thickness St of about 0.05 to about 0.7 mm is required to have a thickness accuracy in the order of sub-microns.
However, in general, the shim 304 formed with a plate produced from a rolled steel plate has an in-plane thickness irregularity as large as several microns due to rolling irregularity. Furthermore, since it is thin, it is difficult to re-machine it for achieving a higher accuracy. Therefore, either in the case of the slit die 301, a lip gap accuracy in the order of sub-microns cannot be achieved.
In FIG. 13, the slit die 401 comprises a right lip 402 and a left lip 403. Both the lips 402 and 403 have a butt interface 415 at their upper portions. The inner lip face 420 of the right lip 402 is positioned with a position difference distance L kept from the butt interface 415. The inner face 421 of the left lip 403 is in the same plane as that of the butt interface 415 and forms a flat lip. Between the inner lip face 420 of the right lip 402 and the inner face 421 of the left lip 403, a lip gap 412 is formed.
The lip gap 412 has a lip gap width L equal to the position difference distance L. This slit die 401 is disclosed in JP 10-146556 A or JP 10-151395 A. In the slit die 401 with this constitution, the lip gap width L of the lip gap 412 is equal to the position difference distance L established in the lip 402. Therefore, for achieving a lip gap accuracy in the order of sub-microns, it is necessary that the position difference between the butt interface 415 and the inner lip face 420 of the lip 402 is formed at a high finishing accuracy in the order of sub-microns.
However, it is difficult to finish a long and large part with a large area like a lip at an accuracy in the order of sub-microns by means of machining using a publicly known grinder, etc. or manual lapping. Therefore, even in the slit die 401, a lip gap accuracy in the order of sub-microns cannot be achieved.
On the other hand, as a die coater using any of these slit dies, known is a die coater comprises a table capable of reciprocating and a coating head (slit die) having a downward discharge opening. In this die coater, a glass substrate is sucked and held on the table, and subsequently, the glass substrate is moved together with the table right under the coating head, when a coating liquid is discharged from the discharge opening of the coating head, to continuously form a coating film of the coating liquid on the glass substrate. This die coater is disclosed in JP 6-339656 A.
In this die coater, since a substrate is coated each by each, the coating methods at the coating start portion and the coating end portion of each substrate are important for enhancing the thickness accuracy of the coating film on the entire substrate. For the coating start portion, available is a method of controlling the relation between the action of the coating liquid feed pump and the action of the substrate. This method is disclosed in JP 8-229482 A.
In another method, preliminary coating from a die to a roll is performed to form bead of the coating liquid between the die and the roll, and the die is moved together with the bead toward the substrate, to start regular coating on the substrate. This method is disclosed in JP 2001-310147 A.
Furthermore, in a method for preventing the thickness at the coating start portion from becoming large, the clearance between the substrate and the die is controlled in interlock with the discharge of the coating liquid and with the horizontal movement of the die to the substrate. This method is disclosed in JP 2002-113411 A.
Among the above-mentioned coating start methods, the method in which the regular coating on a substrate is started after preliminary coating from a die to a roll has such disadvantages that (i) extra equipment is needed to raise the cost, (ii) extra action is needed to make the tact longer, not allowing productivity enhancement, (iii) a slight amount of the coating liquid remains at the tip of the die discharge opening after preliminary coating on the roll, and since the remaining amount is not constant, the thickness of the coating film at the coating start portion varies and is not stable, and (iv) the preliminary coating increases the amount of the coating liquid not used for the regular coating, to raise the cost.
On the other hand, in the case where a coating method without preliminary coating is used, if a coating liquid using a highly volatile solvent is applied to form a wet thickness of 20 μm or less, as shown in FIG. 16A, several non-coated spots 803 where no coating film 802 is formed can occur in the width direction of the substrate B at the coating start portion (head portion) 801. For this defect, the following causes can be considered: (i) the area at and near the discharge opening of the die is cleaned before coating, to keep the coating start portion 801 always in the same state, and in this case, the coating liquid inside the die near the discharge opening is brought away to form voids in the die, or (ii) within the short time after cleaning of the area at and near the discharge opening of the die till coating, it can happen that the solvent of the coating liquid existing in the area at and near the discharge opening is evaporated to form voids in the die near the discharge opening depending on the evaporated amount, and that as a result, the voids not filled with the coating liquid exist in the die and are transferred as they are onto the coating start portion 801 of the substrate B as the non-coated spots 803.
This phenomenon is very unlikely to occur if the wet coating thickness is more than 20 μm. The reason is considered to be that since the rate of the voids to the discharged amount of the coating liquid is small, the voids, even if formed, do not affect the coating state. On the contrary, in the case where preliminary coating is performed, since the voids near the discharge opening are extruded in the stage of preliminary coating, voids do not exist in the die when regular coating is performed. So, the disadvantage that non-coated spots 803 are formed at the coating start portion 801 does not occur.
An object of the invention is to solve the problems of the prior art. The object of the invention is to provide a slit die that allows a lip gap accuracy in the order of sub-microns to be easily achieved. The slit die of the invention allows a uniform coating film to be formed with a very high coating thickness accuracy of 3% or less even if no special adjustment is performed after the die has been assembled.
Another object of the invention is to provide a method and apparatus for producing a substrate with coating films using the slit die.
The substrate with coating films produced by the invention can be preferably used as a member for a color liquid crystal display, or as a member for a plasma display.
The invention allows a coating film with a uniform thickness to be easily formed over the entire surface of a substrate without performing preliminary coating irrespectively of the coating liquid used and irrespectively of the coating thickness. The invention allows the shortening of tact time and the decrease in the amount of wasted coating liquid and allows the production cost to be reduced in the production of a substrate with coating films.